We describe how to perform fast quantum limited read-out of a solid state qubit biased at its degeneracy point. The method is based on homodyne detection of the phase of a microwave signal reflected by a slow oscillator coupled to the qubit. Analyzing the whole quantum read-out process, we find that the detection is indeed quantum limited and that this limit may be reached even using a resonance circuit with a low quality factor, thus enabling the use of short measurement pulses. As an example, we discuss in detail the read-out of a Cooper-pair box capacitively coupled to a lumped element LC-oscillator. Furthermore, we give formulas for the backaction while not measuring, and discuss optimal parameters for a realistic design capable of fast (~50 ns) single-shot read-out.
@article{arxiv.cond-mat/0602583,
title = {Fast quantum limited read-out of a superconducting qubit using a slow oscillator},
author = {G. Johansson and L. Tornberg and C. M. Wilson},
journal= {arXiv preprint arXiv:cond-mat/0602583},
year = {2009}
}
Comments
4 pages,2 figures, Corrected typos and added contents. Accepted to Phys. Rev. B(R)